six is the canonical compatibility library for supporting
Python 2 and 3 in a single codebase.
The xrange module was removed in Python 3 and we should
use 'six.moves.range' instead of 'xrange' to make
code compatible with py 2 and 3 as well.
Partially-implements blueprint py3-compatibility
Change-Id: Id054857f320d9eda02bc0b82e5512c0595342290